Originally Posted by
Calchas
When you give a Mach number, is that an air speed referenced against what Mach 1 would be in air at standard temperature and pressure, or is it the "true" Mach number, i.e., referenced against the current sound speed of the fluid through which you travel?
Mach Is referenced against the fluid through which you are travelling as the speed of sound varies with altitude as well as temperature and density. Aircraft only "know" the conditions they are experiencing not academic numbers in the lab Mach is dependent upon local conditions which is why early attempts to break the sound barrier were only made at certain times of day when conditions were optimal.